The present invention is directed generally to a cover and support plate arrangement for a wall mounted device, such as a wall switch, wall mounted display, etc. In particular, as disclosed herein, but not limited thereto, the present invention is directed to a cover plate and support plate apparatus for a wall switch such as a dimmer or dimmer switch, wherein the support plate is affixed to a wall box embedded in a wall and the cover plate is juxtaposed over the support plate. According to one aspect of the invention, there is provided a support plate and cover plate arrangement for wall mounted apparatus of the type having a controllably conductive device in thermal connection with the support plate and in series with a controlled load and wherein the cover plate appears to float from the wall at wide angles of view. According to a second aspect of the invention, there is provided a low profile cover plate and support plate arrangement for standard size dimmer switches. The dimmer switches meet Underwriters Laboratories operating temperature standards for loads that draw up to about 11.67 amps (1400 W at 120 VAC) according to one embodiment and up to 12.5 amps (1500 W at 120 VAC) according to another embodiment. Neither one of the embodiments requires the use of septa on the support plate, which may be of standard size. According to a third aspect of the invention, the support plate or cover plate includes a pair of separately formed, separately attached rails that may be utilized to affix the cover plate to the support plate without the use of tools and further, in the case of a slide type dimmer switch, may be used to capture and guide the slider. According to a fourth aspect of the invention, a slide type dimmer switch includes a slideable operating member operatively coupled to a cam in contact with a viscous lubricant to provide drag and give the slider an improved "feel". According to a fifth aspect of the invention, there are provided means for maintaining the maximum temperature rise of various parts of a low profile dimmer switch having a smooth and continuous dimming range of up to at least 95% of full voltage within the standards specified by Underwriters Laboratories. By "smooth and continuous", it is meant that a controllably conductive means (such as a triac), and not a mechanical break switch, carries the load current throughout the specified dimming range.
U.S. Pat. No. 3,746,923, incorporated herein by reference discloses a support plate for a dimmer switch wherein the support plate is provided with a series of spaced, outwardly facing septa. These septa are the individual "fins" which project outwards from the flat surface of the support plate. The septa increase the ability of the support plate to dissipate thermal energy generated by a dimmer switch. Such type of dimmer switch shall be referred to herein as a "septa dimmer switch". (The word "septa" is used herein in lieu of the "fins" referred to in the U.S. Pat. No. 3,746,923.) According to the U.S. Pat. No. 3,746,923, a pair of septa is also utilized as a guide for a slider that is connected to the operating member of a linear potentiometer. A cover plate snaps over the septa and has an elongated slot through which access to a handle integral with the slider is provided.
U.S. Pat. No. 3,735,020, also incorporated herein by reference, discloses support plates for dimmer switches, including the septa type support plates of the U.S. Pat. No. 3,746,923, that may be gang mounted onto ganged wall boxes each having a width A. The support plates of the U.S. Pat. No. 3,735,020 dimmer switches each have a width of either 1.5A or 2.5A. For simplicity, these are referred to herein as .intg.1.5A" and 2.5A" dimmmer switches. In commercial practice, 1.5A septa dimmer switches of the type disclosed in the U.S. Pat. No. 3,746,923 measure approximately 23/4 inches wide by 41/2 inches long and have an overall surface area (including that of the septa) of approximately 54 square inches. Also, in commercial practice, 2.5A finned dimmer switches of the type disclosed in the U.S. Pat. No. 3,746,923 measure approximately 41/2 inches square and have an overall surface area (including that of the septa) of approximately 91 square inches. In commercial practice, the support plates of both 1.5A and 2.5A septa dimmer switches have been constructed of aluminum.
It is known that the amount of heat generated by a dimmer circuit is related to the forward voltage drop across the dimmer circuit's triac when operating at a specified current, i.e., the lower the voltage drop, the less heat generated by the triac. Triac selection, however, has not been a major design consideration in septa dimmer switches because such dimmer switches are capable of accommodating a relatively wide range of triacs (i.e., triacs with a relatively high forward voltage drop) while still meeting Underwriters Laboratories standards.
Known 1.5A septa dimmer switches are capable of operating loads that draw currents of up to about 8.33 amps while meeting Underwriter's Laboratories (U.L.) operating temperature standards. Thus, 1.5A septa dimmer switches have been commercially embodied as two separate models, one designed for loads that draw up to 5 amps (600 W at 120 VAC) and another designed for loads that draw up to 8.33 amps (1000 W at 120 VAC). Both have utilized Jedec T0220 type triacs. The 5 amp capacity, 1.5A septa dimmer switch has utilized 25 amp continuous current rated triacs having a forward voltage drop as low as about 1.1 volts at 5 amps forward current. The 8.33 amp capacity, 1.5A septa dimmer switch has utilized 25 amp continuous current rated triacs having a forward voltage drop as low as about 1.2 volts at 8.33 amps. In a 5 amp capacity, 1.5A septa dimmer switch, the triac is mounted on a strip of heat conducting, electrically nonconductive glass tape interposed between the triac and the rear surface of the aluminum support plate. The glass tape and aluminum surface are baked together in well-known manner and the triac is mounted to the glass tape with room temperature vulcanizing rubber such as RTV.RTM.. In an 8.33 amp capacity, 1.5A septa dimmer switch, a small copper strip measuring about 1.23 inches by 0.54 inch is eyeleted to the rear surface of the aluminum support plate (a thermally conductive grease such as Dow Corning 340 Silicone Heat Sink Compound is first applied to the contacting surfaces), one side of a beryllium oxide disk is soldered to the copper strip and the triac is soldered to the other side of the beryllium oxide disk.
Similarly, known 2.5 A septa dimmer switches are capable of operating loads that draw currents of up to about 16.67 amps (2000 W at 120 VAC) while meeting U.L. operating temperature standards. Again 2.5A septa dimmer switches have been commercially embodied as two separate models, one designed for loads that draw currents of up to about 12.5 amps (1500 W at 120 VAC) and another designed for loads that draw currents of up to about 16.67 amps (2000 W at 120 VAC). The commercial embodiment of a 12.5 amp capacity, 2.5A septa dimmer switch has utilized press fit type 25 amp continuous current rating triacs having a forward voltage drop as low as about 1.25 volts at 12.5 amps forward current. The commercial embodiment of a 16.67 amp capacity 2.5 septa dimmer switch has utilized press fit type 40 amp continuous current rating triacs having a forward voltage drop as low as about 1.1 volts at 16.67 amps forward current. In the case of the 12.5 amp capacity 2.5A septa dimmer switch, the triac is mounted in the same manner as in the 8.33 amp capacity 1.5A septa dimmer switch, i.e., with a small copper strip measuring about 1.23 inches by 0.54 inch, thermally conductive grease and a beryllium oxide disk. In the case of the 16.67 amp capacity 2.5A septa dimmer switch, triac mounting is the same except that it is known to use a large copper pad measuring about 2.8 inches by 1.7 inches instead of a small copper strip to improve thermal performance.
The above described septa dimmer switches have been manufactured and sold by the assignee of the present application under names such as NOVA and CENTURION. A desirable feature of such septa dimmer switches is that they are capable of continuous and smooth dimming up to at least 95% full voltage while meeting U.L. standards. Septa dimmer switches have also been manufactured and sold by Prescolite Controls, Carrolton, Tex. under the names PRESET and PRESET n-Touch. The devices manufactured by Prescolite Controls are said to be covered by U.S. Pat. No. 4,455,546.
A non-septum dimmer switch having a support plate less than 1.5A in width for controlling loads that draw up to 8.33 amps (1000 W at 120 VAC) while meeting U.L. standards is known and is manufactured by Power Controls Corporation, San Antonio, Tex. and sold under the name "Tog-L-Dim-R". Such dimmer switch, however, is not capable of full range dimming but instead utilizes a mechanical on-off switch to carry full load current when the dimmer switch has been set about 63% of full voltage. The manufacturer claims continuous dimming "through 90%" but tests on actual commercial units show that continuous dimming occurs only up to the above mentioned percentage of full voltage. Thus, a drawback of such dimmer switch is that it is not capable of smooth and continuous dimming over substantially full range. The device manufactured by Power Controls Corporation is said to be covered by U.S. Pat. Nos. 3,990,033 and 4,085,399.
Another non-septum dimmer switch for controlling loads that draw up to 8.33 amps is manufactured by the assignee hereof under the designation D1000 and is capable of substantially full range dimming. However, such dimmer switch has been manufactured with a 2.5A width support plate (having an overall surface area of approximately 42 square inches) to enable it to meet U.L. operating temperature standards. Thus, this dimmer switch suffers from the drawback of requiring a large support plate.
Heretofore, it was believed that a large surface area provided by either a large non-septum support plate or deep septa disposed on the support plate, was required for 8.33 amp and higher capacity dimmer switches to sufficiently dissipate the maximum heat generated by the dimmer circuitry and meet U.L. standards. A major portion of the heat is generated by the triac in the dimmer circuit and, as mentioned, the amount of heat generated by the triac is related to the triac's forward voltage drop at operating current.
A disadvantage of the cover plate and support plate arrangement disclosed in the U.S. Pat. No. 3,746,923 is that, due to the high outwardly projecting septa, the cover plate is distantly spaced from the support plate and appears to float from the wall at only very narrow angles of view. When viewed at wider angles, it is apparent that the cover plate is spaced from the support plate and the wall by a significant distance and the metal septa on the exterior sides are clearly visible. The assignee has manufactured and sold low profile wall mounted apparatus including a support plate with a cover plate that appears to float from the wall at wide angles of view under the names AURORA and VERSAPLEX. However, neither such device houses circuitry that generates substantial amounts of heat. Moreover, neither such device comprises a chimney, as herein described.
Experimentation with novel heat transfer techniques has shown that a large surface area on the support plate is not required for operating loads that draw currents of up to about 12.5 amps while meeting U.L. standards. In fact, on 1.5A and 2.5A width support plates the septa may be removed entirely. The present invention takes advantage of this finding to provide a low profile support and cover plate arrangement that may be used in connection with dimmer switches and other circuitry and that meets U.L. operating temperature standards.
The present invention also takes advantage of findings relating to the use of rails in wall mounted apparatus generally and to the use of viscous lubricants in slide type dimmer switches. The '923 patent teaches septa, including a pair of that serve as rails for a slider, integrally extruded from a support plate. In commercial practice, however, integrally extruded rails have proven to be problematic, primarily due to difficulties in aligning the septa and maintaining the septa and rails within prescribed tolerances. Often, it has been necessary to include an additional costly manufacturing process to align the septa within specified tolerance. A slide dimmer switch manufacturing by Leviton Manufacturing Co., Little Neck, N.Y. under the name "Decora" utilizes a pair of non-extracted plastic rails to guide a slider. But, the rails are integral with each other--they are part of a molded plastic unit comprising two rails, two connecting members and an integral backplate. Thus, the Leviton unit does not permit easy adjustment of rail spacing during manufacture and hence suffers from problems similar to those associated with extruded rails.
It has also been known to grease the rails of a slide dimmer switch to reduce friction between the slider and rails, but this approach is also problematic. One such problem is that the greased rails accumulate dust and other particles, thus defeating the purpose of the grease. Another problem is that the installer may contact the grease and inadvertantly rub it into the cover plate, thereby leaving an unsightly appearance. The present invention overcomes these and other problems associated with prior art dimmer switches.